In conventional suction pipe systems in the papermaking industry it is standard to use a suction pipe with an elongated slot in alignment with a felt. The suction pipe is positioned so that the felt passes over the slot and when the appropriate pressure differential is produced the felt will be dewatered, the water being drawn into the suction pipe and directed to an appropriate collection location. A conventional separator is used when water is to be separated from air in the system and the air is exhausted in a well known manner.
There are several basic types of vacuum pumps used for providing the most efficient vacuum system for different types of papermaking machine applications. Three basic types of vacuum pumps used in the paper industry are the liquid ring pump, the positive displacement pump, and the centrifugal exhauster, sometimes called a blower. Each type has its advantages and disadvantages over one another, plus different maximum efficiency values on air flow versus vacuum settings. Therefore, it is important to select not only a particular type of vacuum pump for a given application, but also with size, port openings, number of stages, and other criteria, for the lowest horsepower for unit air flow requirement. Lower horsepower naturally reduces manufacturing, assembly and use costs as well as producing lower energy consumption which is of extreme concern today.
Other factors that always have to be considered in the selection of a vacuum pump system besides the lower horsepower requirements are purchase price, total installation cost, maintenance, seal water requirements, amount of liquid with incoming air flow, and presence of contamination such as solids or fibers. In other words, one type of vacuum pump may look good from a horsepower standpoint, but because of the above other considerations, may not be practical or the total system cost is much more expensive than using another type pump.
In considering the above parameters, an important balancing criteria is based upon sufficient power to permit the use of a felt for dewatering purposes over the longest possible time before replacement is required. It is well known that the felt will wear over a period of time in use and will ultimately have to be replaced. However, the felt also undergoes a reduction in permeability as it is used over a period of time for dewatering purposes. This reduction in permeability naturally affects the efficiency of the dewatering system. Consequently, it has been the policy to use a vacuum pump of substantial horsepower in the dewatering system so that the felts can be used for a longer period of time even after its permeability has been substantially reduced during use. It can be readily observed that the larger horsepower vacuum pump is considerably oversized for the system when the felt is new causing the system to be inefficient and expensive during a substantial portion of the time a new felt is employed until the permeability has been reduced sufficiently for the additional horsepower to be needed.
Alternatively, felts can be more frequently replaced but this is also a costly and time consuming procedure which is undesirable in the industry.
It should also be noted that even with the oversized vacuum pump in regard to horsepower, the additional horsepower is often not sufficient to effectively dewater with the use of a single suction pump and a fixed slot width. It has been shown that increased dwell time is also an effective means of efficiently dewatering as well as increasing the pressure differential.